1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to process control of charging, exposure, and developing apparatus in photocopiers and like devices.
2. Description of the Prior Art
Photocopiers, electrophotographic printers, and like devices, all need the ability to maintain proper control over the image producing apparatus to assure high quality, hardcopy outputs. One means for controlling a variety of parameters within the apparatus is using a density or process control patch positioned on the photosensitive or charge-retaining member of the apparatus. The control patch is usually a small area located on the photosensitive member outside the region or frame allocated for the normal image information which will be transferred to the hard copy output medium. The control patch is developed by toner application, and reflected light from the developed patch gives an indication of the density of the developed image and allows for changes in the operating parameters to improve the image quality. The toner concentration, the charge on the photosensitive member, and the amount of exposure from the printhead or exposing device, are all parameters which can be changed according to the information received from the circuitry which looks at the amount of light reflected from the control patch.
In optical copiers which create the image on the photosensitive member directly from the original document through a set of lenses, the process control patch is usually created by localized control of the light source, using a fixed spot or "original" patch located near the area where the original document is placed, or by others methods, such as localized charge control on the photosensitive member. Electronic copiers offer new and different methods for creating the control patch on the photosensitive member. In electronic copiers, the image to be created on the photosensitive member is defined by a modulated light beam which moves across the surface of the photosensitive member. Laser scanners and light emitting diode (LED) array printheads are two examples of writing devices for electronic copiers and printers. In such apparatus, the control patch is usually created by sending a known pattern of data to control the modulation of the light-emitting elements in the writing head. In the case of an LED printhead array which uses a line of LED's across the whole page, the data is driven into the LED printhead in a serial fashion for the complete line. Once a line of data has been loaded, the printhead LED's are turned on or off, depending upon the characteristics of the data loaded therein.
Process control patches are usually small compared to the overall frame or image area on the photosensitive member. A control patch one-quarter inch square can provide the necessary area to reflect light for the process control measurements. However, since the data loaded into the LED printhead forms a whole line across the photosensitive member, the conventional way of printing a process control patch requires much more memory than actually needed. A one-fourth inch square process control patch positioned on a photosensitive member with a resolution of 400 dpi (dots per inch) contains only 10,000 pixels. Assuming a binary system where each pixel is either on or off, 10,000 bits of memory is needed to store the pixel information for this control patch. However, because conventional practice loads the whole line for the printhead, 480K bits are actually needed in the memory storage to represent the control patch, assuming the printhead is 12 inches long. Providing such a large amount of memory according to the prior art to produce such a small patch area makes for inefficient use of memory space and, just as important, makes it much more difficult to integrate the printhead driver electronics onto an ASIC (Application Specific Integrated Circuit). Therefore, it is desirable, and it is an object of this invention, to provide circuitry for generating a process control patch which does not require the large amount of memory used by circuitry constructed according to conventional practices.
Typically, process control patches are created at the highest contrast or density level, that is, with all of the pixels in the patch completely black. While this is sufficient in many cases to provide the information needed to control the variables of the process, sophisticated control algorithms and monitoring circuitry is becoming available which can make better use of the control patch when it is printed at some level between the lightest and the darkest shades or density available. In order to vary the density of the control patch according to the prior art, it has been necessary to change the memory which has been dedicated to the data for creating the patch. As previously stated, this memory can be considerable in size and, therefore, changing the density of the control patch is a significant undertaking and may require an appreciable amount of reprogramming and data transfer. Therefore, it is desirable, and it is another object of this invention, to provide a process control patch generator which can easily be changed to create control patches having different densities.
Another problem associated with conventional control patch generators involves the placement of the patch on the photosensitive member. Rearranging or moving the location of the patch, or changing the physical size of the patch, according to the prior art, requires major reprogramming and data changes similar to the changes needed for changing the density of the patch, as previously stated. The large memory which is needed, according to the prior art, to provide the small density control patch, requires inefficient use of memory and restricts circuitry design, placement, and changes. Therefore, it is desirable, and it is still a further object of this invention, to provide a control patch generator which can easily be changed to alter the size and location of the control patch on the photosensitive member.